Originally, co-axial cable networks were designed to be one-to-many distribution systems. Later, without changing the in-ground cable plant, the system was modified to accommodate communication in the reverse or upstream direction. The legacy is a system with upstream channels that are noisy, echo-laden, and unique to each user.
Two-way transmission of data over coaxial cable requires a modulator and demodulator (modem) at each end. The modem in the subscriber's residence is called the cable modem. The modem in the cable operator's premises (head end) is part of a larger piece of equipment called the Cable Modem Termination System (CMTS). The modem in the CMTS is referred to as the CMTS modem. An overview of a typical coaxial cable network is provided in FIG. 1.
DOCSIS, which is an acronym for Data Over Cable Service Interface Specification, is an international family of standards developed by CableLabs. The DOCSIS standards govern the operation of both the upstream and down-stream cable channels. In this document, it is the upstream channel that is of interest. The DOCSIS standard allows the CMTS to partition and allocate the spectrum reserved for upstream channels as well as define the packet preamble, packet payload and modulation format on a per-user basis.
The upstream cable channel, as described in the DOCSIS standard, is echo-laden and unique to each user. DOCSIS specifies that each cable modem should pre-equalize its upstream transmissions using a 24-tap linear equalizer. It is the responsibility of the CMTS to determine the required pre-equalizer tap values during the initialization process and to send these values to the cable modems via the down-stream channel. The CMTS demodulator determines the pre-equalizer coefficients for each user of the upstream channel through the use of an adaptive equalizer.
One possible structure for the upstream demodulator is shown in FIG. 2. In the demodulator of FIG. 2, a known preamble sequence is used to train the adaptive equalizer and recover symbol timing, carrier frequency, and phase on a packet-by-packet basis. Since training the equalizer is the most difficult of these functions, the total required preamble length for the upstream packets is largely determined by the convergence time of the equalizer. The tap weights of the adaptive equalizer are commonly updated using an error-driven stochastic descent algorithm, such as the Least Mean Squares (LMS) algorithm. Unfortunately, such algorithms tend to converge relatively slowly, necessitating a long training sequence. A lengthy training sequence is undesirable, as it adds considerable overhead to the upstream channels.